Why nominative instead of accusative with verb "sum"?

Question

Recently I've been learning about the accusative case, in/direct objects and in/transitive verbs. In light of this, consider the phrase:

Nilus fluvius est

I'm interested in the rationale (rationalis?) of why the above uses fluvius instead of fluvium. In particular:

1. is fluvius a direct object? Since "what is" is fluvius, I would thought so.

2. is the verb sum (beside being a copulative verb) a transitive verb? I would have thought so too, since it is a verb that can take an object (I am a human; human being the object taken by the verb to be).

But then why fluvius and not fluvium? If 1 is not true, then what is the function/structure of the object (noun/adjective) next to the copulative verb "to be"?

I can see that when using the verb "sum" (except in its "to have" meaning), the case for the "object" of the verb is in nominative. But I see no reference to this exception when reading about the accusative case (which is supposed to be the case of the direct object next to a transitive verb).

Answer 1

In all Indo-European languages that I know, copulae are intransitive and normally take the nominative. So everything below will apply to other Indo-European languages, too.

It is important to distinguish between objects and other complements. A direct object, which you mean, is always a kind of complement, accompanying a transitive verb; but there are other complements that aren't direct objects and don't require a transitive verb. Note that, in Latin, only direct objects can be passivised (just as in Dutch and German, but unlike English and Greek). Cf.:

I went to Rome: the complement to Rome is not an object, let alone a direct object, but rather a prepositional phrase.

In Italiam ii: same as above.

Romam ii: even though the complement is accusative, it's not a direct object. This can be seen in what happens when we make it passive:

Romam itur a me: normally, a direct object turns into a subject when we make the sentence passive, which does not happen here.

Tibi parceo: "I spare you": the complement is a dative, not an accusative.

Tibi parcitur a me: this is again visible in the passive (unnatural though a me sounds here).

Te video => videris a me: only direct objects become subjects when passivised.

The primary complement of a (finite) copula—or the secondary, if you count the subject as the primary complement—is always a nominative. This applies both to adjectives and substantives:

Canis sum: I am a dog.

Vetus sum: I am old.

Imagine if the adjective had to be in the accusative case: wouldn't that be weird?

Veterem sum.

You would have an adjective sitting there alone in the accusative. It would agree with nothing. So it has to be its own constituent; but it is very strange to have an adjective that clearly describes a property of the subject ("I") but that isn't in the same case and that isn't the same constituent.

So, because the complement of a copula always describes a property or identity of the subject, it has to be in the same case as the subject. This applies as well to the accusative with infinitive:

Dico me canem esse.

Dico me veterem esse.

The complement of a copula doesn't have an independent case: it must always take the same case as that of the 'subject' of the copula even when the latter isn't in the nominative.

Answer 2

In (most? all?) Indo-European languages the verb "to be" is called copulative because it joins words that represent the same thing -- in your case, "Nile" and "river." In English, Jane might answer a phone caller's question, "Is this Jane?" with the grammatically correct "Yes, this is she" -- though, since about 1600, by analogy with the general run of post-verbal pronouns, the tendency has been to answer "Yes, it's me" (see https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=2ahUKEwjyy7GAiLjdAhUnLMAKHV7eBPwQFjACegQICBAC&url=https%3A%2F%2Ffreidok.uni-freiburg.de%2Ffedora%2Fobjects%2Ffreidok%3A8431%2Fdatastreams%2FFILE1%2Fcontent&usg=AOvVaw0zzyriDMYpaOLj5Q71-nSA).

Answer 3

Sum looks like a transitive verb in that there is a subject and an "object" (which isn't really an object). However, it is not a transitive verb. One way that it is often explained is that sum, esse acts like an equals sign ("="). Both sides of the equals sign have to match in case. When you say that Nilus fluvius est, you are saying that the Nile is a river, i.e. it is equivalent to a river. They are therefore interchangeable. Another example is animal canis est. "The animal is a dog." If you were to interchange animal and canis, the sentence would still be correct. "The dog is an animal." You could not do this for any other verb. Here is a simple example: Puer puellam amat. "The boy loves the girl." You can't interchange the boy and the girl because you don't know if the girl loves the boy, you only know that the boy loves the girl. This is just an easy way to understand why sum, esse is different in this respect.

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In more complex sentences, you don't use the nominative. For example, one would translate "I know the boy is handsome" as scio puerum esse pulchrum. esse is once again an equals sign, matching the case of puer, which is accusative as it is the direct object of scio, to the case of pulchrum, which is masculine like puer and also in the accusative.

Answer 4

A less-technical answer that could help the less technical (although it could be less precise):

This happens in most (if not all?) languages with declensions.

One way to see it, put in simple words, is that a so-called copulative verb (like to be) is not a proper action with an object, a recipient and a number of complements/modifiers. It is instead a way of describing/classifying the subject.

Whatever you say as the predicate of a copulative sentence (A is B) is something about the subject, modifying the subject itself. Thus, it is reasonable for the predicate (B) to agree with the subject (A).

It becomes apparent when the predicate is an adjective: in The dog is small, small is qualifying the dog, (almost?) the same as saying the small dog as a subject of a different sentence: the small dog bit my hand.